Idiomatic Node async call with eror handling

asynWithErrorHandling.js

// this is an idiomatic example of a Node async call with callbacks that 
// handle both returned data and error

/***********This is the module defined for later use */

var fs = require('fs');

module.exports = function(file, ext, callback) {
    fs.readdir(file, function(err, data) {
        // if readdir returns error, call callback with error early
        if (err) {
            return callback(err);
        }
        // if readdir returns with data, do some stuff with it
        data = data.filter(function(each) {
            return each.split('.')[1] === ext;
        });
        // then call callback with data (error is null because we already checked)
        return callback(null, data);
    });
};

/***This is the program that calls the module above**/

var module = require('./module.js');

// file path is from enviroment variable passed in through CLI
var file = process.argv[2];
var ext = process.argv[3];

// idiomatic callback functions have error handling
var callback = function(err, data) {
    if (err) {
        return console.error('there was an I/O error');
    } else {
        data.forEach(function(each) {
            console.log(each);
        });
    }
};

module(file, ext, callback);